Many different methods for the preparation of epoxides have been developed. One such method involves the use of certain titanium silicalite materials to catalyze olefin oxidation by hydrogen peroxide. This method is described, for example, in Huybrechts et al., J. Mol. Catal. 71, 129 (1992), U.S. Pat. Nos. 4,824,976 (Clerici et al.) and 4,833,260 (Neri et al.), European Pat. Pub. Nos. 311,983, 190,609, 315,247 and 315,248, Belgian Pat. Pub. No. 1,001,038, Clerici et al., J. Catal. 129,159(1991), and Notari, in "Innovation in Zeolite Material Science," Studies in Surface Science and Catalysts, vol. 37, p. 413 (1988).
However, the outcome of synthetic reactions catalyzed by titanium silicalites is highly unpredictable and seemingly minor changes in reactants and conditions may drastically change the type of product thereby obtained. For example, when an olefin is reacted with hydrogen peroxide in the presence of titanium silicalite the product obtained may be either epoxide (U.S. Pat. No. 4,833,260), glycol ether (U.S. Pat. No. 4,476,327), or glycol (Example 10 of U.S. Pat. No. 4,410,501).
The prior art related to titanium silicalite-catalyzed epoxidation teaches that it is beneficial to employ a hydrogen peroxide solution that does not contain large amounts of water and recommends the use of an organic solvent as a liquid medium for the epoxidation reaction. Suitable solvents are said to include polar compounds such as alcohols, ketones, ethers, glycols, and acids. Solutions in tert-butanol, methanol, acetone, acetic acid, and propionic acid are taught to be most preferred. However, hydrogen peroxide is currently available commercially only in the form of aqueous solutions. To employ one of the organic solvents recommended by the prior art, it will thus be necessary to exchange the water of a typical hydrogen peroxide solution for the organic solvent. This will necessarily increase greatly the overall costs associated with an epoxidation process of this type. Additionally, concentration of hydrogen peroxide to a pure or nearly pure state is exceedingly dangerous and is normally avoided. Thus, it will not be practicable or cost-effective to simply remove the water by distillation and replace it with the organic solvent. Since hydrogen peroxide has a high solubility in and high affinity for water, liquid-liquid extraction of hydrogen peroxide from an aqueous phase to an organic phase will not be feasible. Moreover, many of the solvents taught by the prior art to be preferred for epoxidation reactions of this type such as tert-butanol, acetone, and methanol are water miscible and thus could not be used in such an extraction scheme. An epoxidation process wherein a readily obtained oxidant solution containing hydrogen peroxide and an organic solvent which promotes high yields of epoxide products is employed would thus be of significant economic advantage.
U.S. Pat. No. 5,214,168 proposes an integrated process for epoxide production wherein an aryl-substituted secondary alcohol such as alpha methyl benzyl alcohol is oxidized with molecular oxygen to provide the hydrogen peroxide used in a subsequent epoxidation step. While this process works well when practiced in a batch type mode, it has now been found that certain of the titanium silicalites employed as epoxidation catalysts experience deactivation when such a process is run on a continuous basis. This deterioration in activity and selectivity is believed to be due to the accumulation of oligomeric and polymeric by-products derived from the aryl-substituted secondary alcohol or other species present in the epoxidation reaction mixture. While the deactivated catalyst could be regenerated using known techniques such as solvent washing and/or recalcination, it would be highly advantageous to develop a continuous integrated epoxidation process wherein less frequent catalyst regeneration or replacement is needed.
Example 35 of U.S. Pat. No. 4,833,260 describes a procedure wherein propylene is converted to propylene oxide. An isopropanol/water mixture is reacted with oxygen at 135.degree. C. to afford a mixture containing hydrogen peroxide. The mixture is thereafter used directly in a titanium silicalite-catalyzed epoxidation of propylene without intervening treatment or fractionation. The temperature during epoxidation is carefully maintained at 20.degree. C. by means of a constant temperature bath. Due to the highly exothermic nature of the olefin epoxidation reaction, it is quite difficult to maintain a reaction of this type at room temperature or lower, especially if practiced on a large scale. Even if an effective means of removing heat from the reaction mixture is employed, the utility (cooling) costs associated with such an arrangement will place the process at a distinct competitive disadvantage relative to conventional epoxidation processes. It would thus be highly desirable to be able to operate a titanium silicalite-catalyzed epoxidation using a secondary alcohol-derived hydrogen peroxide stream at superambient temperatures without a significant selectivity penalty.
Another problem associated with the use of an oxidized isopropanol mixture as a source of hydrogen peroxide in a olefin epoxidation reaction catalyzed by titanium silicalite is the potential for forming significant quantities of organic peroxides from interaction of the hydrogen peroxide and the acetone generated by oxidation of the isopropanol. See, for example, Sauer et al., J. Physical Chem. 75, 3004-3011 (1971) and Sauer et al., ibid. 76, 1283-1288 (1972). These organic peroxides have been found to accumulate during isopropanol oxidation, during storage of the oxidate mixture, as well as during olefin epoxidation. The formation of such peroxide species detracts from the selective transformation of an ethylenically unsaturated substrate to epoxide since hydrogen peroxide is being consumed. In addition, the presence of the organic peroxides, some of which may be highly explosive in pure form, complicates the purification and separation steps following epoxidation.